Appendix VI. 1137 opposite end. The important part of this mechanism is the convex mirror E, which receives a ray of light from the acetylene lamp c after reflection by prism D, and projects it upon B. The larger drawings shew the mechanism more clearly. A gun-metal casting F fixed to the box by the seat G, contains the plungers K and L' whose motions represent engine stroke and pressure respectively : the pressure movement being caused through the buckling of a thin diaphragm M by the engine pressure in a small- bore pipe N. The plunger K is actuated by the * reproducer' Q R s T, so called because it imitates the engine stroke, and consisting of a flexible shaft Q, connected at one end to shaft R, and at the other to the crank shaft of the engine. Two equal spur wheels continue the motion to shaft s, upon which is a crank pin and link to rock the lever T. The proportions of these levers must be such as to copy on the average the motion of the crank and connecting rods of the engines usually indicated; and the bearings of the two shafts are contained in the larger bush p, which may be rotated into any required position round the centre s, by the milled wheel u and worm v. The object of this change is the synchronisation of x's phases with those of the engine stroke, which cannot be done till the flexible shaft has taken its drive. The mirror is supported at three points K, L and w, which together form a right-angled triangle ; and the point w is a rigid fulcrum or pivot, but the supports K and L are the springs j and H respectively. The plunger K, pressing behind spring j, moves the mirror in a horizontal plane to represent stroke ; while the plunger L, pressing behind spring H, moves the mirror in a vertical plane, or at right angles to the former move- ment, and indicates pressure. As the deflections of the plate M are not exactly proportional to pressure, the back of the mirror is so curved at the point / that the necessary compensation is affected. The pipe N is connected to a three-way cock, of the usual pattern, on the engine cylinder; and the shaft Q can be connected while the engine is running. When the speed is 1200 revs, per m. in a four-cycle motor, the indicator diagram can be viewed as one picture on account of the persistence of vision. It can then be traced on the plate. But the best way is to insert a photographic plate in a dark slide, and expose for one or two seconds, In this manner Prof.